Diffractive Higgs production

Instrumenting the LHC to measure the outgoing protons in the process pp -> p+X+p opens up new possibilities for physics studies in the Standard Model and beyond. In this talk I first present an overview of the underlying QCD calculation and its uncertainties. I then move on to present a variety of new physics scenarios which one would want to explore via this unique channel. There is a short discussion of the recent CDF data which may provide evidence for the existence of central exclusive jet production at the Tevatron.Comment: 8 pages. Talk presented at "Diffraction 2006

High-t Diffraction

The study of those rapidity gap processes where a large momentum is transferred across the rapidity gap provides an ideal opportunity to understand the gap producing mechanism wholly within the framework of QCD perturbation theory. The current theoretical and experimental status of this `high-t diffraction' is reviewed.The study of those rapidity gap processes where a large momentum is transferred across the rapidity gap provides an ideal opportunity to understand the gap producing mechanism wholly within the framework of QCD perturbation theory. The current theoretical and experimental status of this `high-t diffraction' is reviewed

Parton branching at amplitude level

We present an algorithm that evolves hard processes at the amplitude level by dressing them iteratively with (massless) quarks and gluons. The algorithm interleaves collinear emissions with soft emissions and includes Coulomb/Glauber exchanges. It includes all orders in $N_{\mathrm{c}}$, is spin dependent and is able to accommodate kinematic recoils. Although it is specified at leading logarithmic accuracy, the framework should be sufficient to go beyond. Coulomb exchanges make the factorisation of collinear and soft emissions highly non-trivial. In the absence of Coulomb exchanges, we show how factorisation works out and how a partial factorisation is manifest in the presence of Coulomb exchanges. Finally, we illustrate the use of the algorithm by deriving DGLAP evolution and computing the resummed thrust, hemisphere jet mass and gaps-between-jets distributions in $e^+ e^-$.Comment: 54 pages, minor changes in version

Diffraction 2006: Theoretical Summary

This paper presents a summary of the theoretical presentations to the international workshop "Diffraction 2006". The range of topics covered during the workshop was quite broad and this summary is therefore somewhat selective covering recent developments in BFKL physics, exclusive processes, saturation dynamics, DIS and structure functions and coherence in QCD.Comment: 16 pages. Talk presented at "Diffraction 2006

Precise Predictions for Higgs Production in Neutralino Decays in the Complex MSSM

Complete one-loop results are obtained for the class of processes chi^0_i->chi^0_j h_a in the MSSM where all parameters entering this process beyond lowest order are allowed to have arbitrary CP-violating phases. An on-shell renormalisation scheme is worked out for the chargino-neutralino sector that properly takes account of imaginary parts arising from complex parameters and from absorptive parts of loop integrals. The genuine vertex contributions to the neutralino decay amplitudes are combined with two-loop propagator-type corrections for the outgoing Higgs boson. In this way the currently most precise prediction for this class of processes is obtained. The numerical impact of the genuine vertex corrections is studied for several examples of CP-conserving and CP-violating scenarios. We find that significant effects on the decay widths and branching ratios are possible even in the CP-conserving MSSM. In the CP-violating CPX benchmark scenario the corrections to the decay width are found to be particularly large, namely, of order 45% for a Higgs mass of 40 GeV. This parameter region of the CPX scenario where a very light Higgs boson is unexcluded by present data is analysed in detail. We find that in this parameter region, which will be difficult to cover by standard Higgs search channels at the LHC, the branching ratio for the decay chi^0_2->chi^0_1 h_1 is large. This may offer good prospects to detect such a light Higgs boson in cascade decays of supersymmetric particles.Comment: 24 pages, 6 figure

Coulomb gluons and the ordering variable

We study in detail the exchange of a Coulomb (Glauber) gluon in the first few orders of QCD perturbation theory in order to shed light on their accounting to all orders. We find an elegant cancellation of graphs that imposes a precise ordering on the transverse momentum of the exchanged Coulomb gluon.Comment: 28 pages, 12 figure

Ordering multiple soft gluon emissions

We present an expression for the QCD amplitude for a general hard scattering process with any number of soft gluon emissions, to one-loop accuracy. The amplitude is written in two different but equivalent ways: as a product of operators ordered in dipole transverse momentum and as a product of loop-expanded currents. We hope that these results will help in the development of an all-orders algorithm for multiple emissions that includes the full colour structure and both the real and imaginary contributions to the amplitude.Comment: v4: 3 pages; 1 figure, 4 references and small clarifications added. Version accepted for publication in Physical Review Letter

Diffractive production of high pTp_{T} photons at a future linear collider

We examine the prospects for studying the diffractive production of high pt photons in the process gamma+gamma -> gamma+X at a future linear collider operating in both ee and gamma-gamma modes. The high luminosity associated with a linear collider make it the ideal place to measure this process